Secondary, or rechargeable, lithium ion batteries are used in many stationary and portable devices, such as those encountered in the consumer electronic, automobile, and aerospace industries. The lithium ion class of batteries has gained popularity for various reasons including a relatively high energy density, a general nonappearance of any memory effect when compared to other kinds of rechargeable batteries, a relatively low internal resistance, and a low self-discharge rate when not in use.
A lithium ion battery generally operates by reversibly passing lithium ions between a negative electrode (i.e., anode) and a positive electrode (i.e., cathode). The negative and positive electrodes are situated on opposite sides of a microporous polymer separator that is soaked with an electrolyte solution suitable for conducting lithium ions. Each of the negative and positive electrodes is also accommodated by a current collector. The current collectors associated with the two electrodes are connected by an interruptible external circuit that allows an electric current to pass between the electrodes to electrically balance the related migration of lithium ions. Further, the negative electrode may include a lithium intercalation host material, and the positive electrode may include a lithium-based active material that can store lithium ions at a higher electric potential than the intercalation host material of the negative electrode. The electrolyte solution may contain a lithium salt dissolved in a non-aqueous solvent.
A lithium ion battery, or a plurality of lithium ion batteries that are connected in series or in parallel, can be utilized to reversibly supply power to an associated load device. A brief discussion of a single power cycle beginning with battery discharge follows.
At the beginning of a discharge, the negative electrode of a lithium ion battery contains a high concentration of intercalated lithium while the positive electrode is relatively depleted. The establishment of a closed external circuit between the negative and positive electrodes under such circumstances causes the extraction of intercalated lithium from the negative anode. The extracted lithium atoms are split into lithium ions and electrons as they leave an intercalation host at the electrode-electrolyte interface. The lithium ions are carried through the micropores of the interjacent polymer separator from the negative electrode to the positive electrode by the ionically conductive electrolyte solution while, at the same time, the electrons are transmitted through the external circuit from the negative electrode to the positive electrode (with the help of the current collectors) to balance the overall electrochemical cell. This flow of electrons through the external circuit can be harnessed and fed to a load device until the level of intercalated lithium in the negative electrode falls below a workable level or the need for power ceases.
The lithium ion battery may be recharged after a partial or full discharge of its available capacity. To charge or re-power the lithium ion battery, an external power source is connected to the positive and the negative electrodes to drive the reverse of battery discharge electrochemical reactions. That is, during charging, the external power source extracts the lithium present in the positive electrode to produce lithium ions and electrons. The lithium ions are carried back through the separator by the electrolyte solution, and the electrons are driven back through the external circuit, both towards the negative electrode. The lithium ions and electrons are ultimately reunited at the negative electrode, thus replenishing it with intercalated lithium for future battery discharge.
The ability of lithium ion batteries to undergo such repeated power cycling over their useful lifetimes makes them an attractive and dependable power source.